-о/-е/-ё

In Russian, every singular noun has a grammatical gender (plural-only nouns like ножницы - scissors do not). Neuter nouns primarily refer to inanimate objects or abstract ideas, though there are animate exceptions like животное (animal). Additionally, some neuter nouns like животное and мороженое (ice cream) end in -ое or -ее because they are actually adjectives functioning as nouns and decline like adjectives.

Common Endings:

• -о: Used after hard consonants (e.g., окно - window). It is also used after sibilants (ж, ч, ш, щ) and ц only if the ending is stressed (e.g., лицо - face, плечо - shoulder).
• -е: Used after soft consonants (e.g., море - sea, платье - dress) and after sibilants or ц if the ending is unstressed (e.g., солнце - sun, полотенце - towel).
• -ё: The stressed variant of the soft ending -е (e.g., бельё - laundry, ружьё - gun).
• -ие: A very common ending for abstract nouns and buildings (e.g., здание - building, упражнение - exercise). Note that in the Prepositional case, these nouns take the ending -и instead of the standard -е (e.g., в здании).

Special Cases:
There is a small group of nouns ending in -мя (like время - time, имя - name) that are neuter. When declining these -мя nouns into other cases, they add the suffix -ен- to the stem (e.g., Genitive singular: времени), with the exception of the Genitive plural forms семян (from семя) and стремян (from стремя). Another exception is the irregular noun дитя (child). Additionally, many borrowed words ending in -о, -е, -и, -у, and -ю like метро, кино, такси, рагу, and меню are neuter and indeclinable. However, be careful not to over-apply this rule, as there are masculine loanword exceptions such as маэстро (maestro), атташе (attaché), and шимпанзе (chimpanzee), as well as кофе (coffee), which is strictly masculine in the literary norm but accepted as neuter in colloquial speech. There are also feminine indeclinable loanwords ending in vowels, which usually take their gender from their generic Russian equivalent (e.g., авеню - avenue [улица], салями - salami [колбаса], кольраби - kohlrabi [капуста]). There are also native Russian masculine nouns ending in -о or -е, such as diminutives (like домишко - little house), augmentatives (like домище - huge house, волчище - big wolf, дружище - buddy), and words like подмастерье (apprentice).

Agreement and Cases:
When using a neuter noun in the singular, you must use the neuter forms of other words:

• Pronouns: это, оно, моё, твоё, наше, ваше.
• Adjectives: usually end in -ое or -ее (e.g., новое, синее).
• Past Tense: verbs end in -ло (e.g., окно было).
• Accusative Case: For all neuter nouns, the singular Accusative always looks exactly like the Nominative, regardless of whether the noun is animate or inanimate.

Plural Forms:
In the plural, neuter nouns typically take the ending -а or -я (e.g., окна, моря). Note that gender agreement disappears in the plural, so adjectives and verbs will use the standard plural forms for all genders. However, there are important exceptions: some nouns have irregular plurals ending in -и, -ья, or -еса (e.g., яблоко → яблоки, дерево → деревья, чудо → чудеса, небо → небеса), and nouns ending in -мя insert -ен- in the plural (e.g., имя → имена). Additionally, while inanimate neuter nouns have identical Nominative and Accusative plural forms, animate neuter nouns follow the Accusative = Genitive rule in the plural (e.g., вижу животных).